I started with the back cushions. There used to be three long cushions on this sofa, and none were practical for pillow forts or useful as floor cushions. I pulled the fluff out and used some cotton muslin to make four large pillow forms. These are just simple squares, 19 x 19 inches.
When the forms were all stitched up, I used the canvas (painter's tarp) to sew a couple of 20 x 20 squares together on three sides to make pillow covers. I hemmed the fourth side, but left it open. With the open end at the bottom and a snug fit, the open bottoms aren't too noticeable.
It's not perfect, but it's a nice change for spring. The brown was just looking kind of dirty all the time; this helps. The new covers are really easy to remove and wash, and the canvas takes dye well if I decide the color needs to change. Overall, I'd call this one a success!